Interactive Effects of Source Taxa and Charring Temperature on Pyom Stability, Biological Reactivity, And Influence on Native Soil C In a Northern Forest Soil

Christy D Gibson, Purdue University

Abstract

Fire is a major driver of carbon (C) and nitrogen (N) cycling in forests, releasing significant quantities of greenhouse gases, soot, and aerosols while simultaneously depositing pyrogenic organic matter (PyOM) onto forest soil. PyOM, the product of the incomplete combustion of biomass, makes up a significant portion of soil organic carbon (SOC) (~5 – 45%), The condensed aromatic structure of PyOM imparts a resistance to weathering and decay thus, PyOM can persist in soil for centuries to millennia -much higher than its unpyrolyzed source material. PyOM is also able to impact the turnover of soil organic C. ^ While many studies have linked PyOM production and source material to its biological reactivity, few studies have been able to determine the stability and reactivity of well-characterized PyOM, its effect on rate of native soil C (NSC) mineralization, or the effect of weathering of PyOM on its biological reactivity in field or laboratory decay studies. Addressing these knowledge gaps are particularly important as fire frequency and intensity are expected to increase in boreal and temperate ecosystems which are vulnerable to vegetation shifts and climate change. ^ This dissertation explored the interactive effects of source taxa and charring temperature on PyOM and NSC turnover as well as the effects of photo-oxidative weathering of PyOM on microbial and fungal response in soil and single – fungal culture laboratory incubation experiments which utilized 13 C enriched wood and PyOM material to track the fate of PyOM through soil and culture medium. The resulting data show that weathered PyOM stimulated fungal respiration, enzymatic activity and increased medium C mineralization in malt – derived growth medium. Photo weathering of PyOM reduced the size of its fast cycling C pool, increased microbial oxidative enzyme activity and induced a decrease in NSC respiration rates. ^ The addition of two chemically and structurally distinct PyOM, produced at 200, 300, 450, and 600 °C, as well as its precursor wood sources, jack pine and red maple to a Northern forest soil, resulted in a net reduction in NSC mineralization, decreased fast cycling C turnover, and resulted in enzyme activities and mineralization rates that reflected the onset of thermochemical transitions (PyOM produced at 300 °C) for both taxa. These effects on the interaction of PyOM and NSC were counteracted by the presence of a labile C source, sucrose, in JP treatments. Physiochemical characteristics, specifically, low molecular weight C and thermal stability were determined to be the strongest predictors of fast cycling PyOM C mineralization and turnover of the more resistant, slow cycling PyOM C pool. These results in general indicate that PyOM physiochemical characteristics are useful predictors in determining the relative stability of PyOM and biological reactivity in soil.^

Degree

Ph.D.

Advisors

Timothy R. Filley, Purdue University.

Subject Area

Biogeochemistry|Soil sciences|Geochemistry

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